That cage size will do temperorary but it sounds like long term, it may be too small, as the hides and water bowl would immediately take up most of the floor space, hence I recommend starting with a 20 gallon long (29 inch).
The heat mat is fine. Make sure it is plugged to a thermostat.
Not all Leos will eat crickets. Some take them well, others won't. Mine do not like them and I could not get him to change his mind. I would start small and a variety to see what it will accept before taking in 1000 crickets. I feed mine primarily mealworms plus butterworms, small roaches, black soldier fly larvae, waxworms and small hornworms. Again, try a variety and see what works and then order in bulk to save yourself the money and aggravation.